1. Field of the Invention
The present invention relates to an apparatus for diagnosing a malfunction in an evaporated fuel purge system, in which an evaporated fuel in an internal combustion engine is made to adhere to an absorbent in a canister. Then the evaporated fuel is purged to an intake system in the internal combustion engine under predetermined operating conditions.
2. Description of the Related Art
In an internal combustion engine, a sealed evaporated fuel purge system provided to prevent the evaporated fuel in the fuel tank from escaping into the atmosphere, is sealed so that evaporated fuel is contained within a canister wherein it adheres to an absorbent. Thereafter, the adhered evaporated fuel is purged into an intake passage through a purge control valve at a predetermined timing.
In this kind of internal combustion engine equipped with the evaporated fuel purge system, when an evaporated fuel passage is damaged or pipes are disconnected, the evaporated fuel escaped into the atmosphere. To prevent this, it is necessary to detect whether or not any malfunction in the evaporated fuel purge system has occurred. For this purpose, generally, the internal combustion engine equipped with the evaporated fuel purge system is provided with a malfunction diagnosis apparatus.
In a conventional malfunction diagnosis apparatus for an evaporated fuel purge system, an air valve is provided to an air induction port of a canister. In this kind of malfunction diagnosis apparatus for evaporated fuel purge systems, when diagnosing malfunctions, the air valve is closed so as to seat the canister from the atmosphere, the purge control valve is held open at a predetermined degree of opening so as to introduce a negative pressure from the intake pipe to the system to maintain the purge system at a predetermined negative pressure. Then, the purge control valve is closed and pressure changes after the purge valve is closed are detected. When the degree of the pressure change is larger than the judging value, it is judged that the malfunction such as failure has occurred in the system. When the value is smaller than the judging value, it is judged that there is no malfunction. An example of this pressure action when diagnosing malfunctions is shown in FIG. 8.
Thus, the malfunction diagnosis apparatus for evaporated fuel purge systems provided with an air valve is advantageous in that diagnosis can be quickly made in the negative pressure since the negative pressure is introduced by sealing the canister from the atmosphere.
Moreover, in the malfunction diagnosis apparatus for evaporated fuel purge systems including an air valve, there is one type in which the canister and the fuel tank are communicated as one system so as to conduct the malfunction diagnosis simultaneously, while there is another type in which a tank internal pressure control valve is provided between the canister and the fuel tank and the malfunction diagnosis is conducted separately using a tank internal pressure control valve in the tank side and in the canister side.
In this type of apparatus equipped with an tank internal pressure control valve, the diagnosing time can be shortened because the malfunction diagnosis is conducted in each closed space of small capacity by separating two systems of the tank side and the canister side. Thus, the purge interruption time can be shortened. Therefore, a reduction in evaporated fuel processing ability is decreased and when the purge is restarted after the malfunction diagnosis, the air fuel ratio will be more properly controlled.
Incidentally, it is necessary to maintain the system subjected to the diagnosis at a predetermined target negative pressure when diagnosing malfunctions so as to increase accuracy of the malfunction diagnosis. If the negative pressure in the system changes while a malfunction is diagnosed, the pressure changes may vary even when the same malfunction is diagnosed. Even when the judging value and the judging time are set in the same conditions, a different judging result may be obtained.
However, the degree of valve opening of the purge control valve is not constant, it changes with the operating conditions of an engine. Thus, if the purge control valve is closed simply after opening the valve at a fixed time, the target negative pressure cannot
As disclosed in Japanese Patent Laid-Open Publication No. 6-147031, the quantity of purge flow was detected and an introduction time of the negative pressure (i.e., time from closing the air valve to closing the purge control valve) was changed in accordance with the quantity of purge flow.
However, the above technique of changing the introduction time of the negative pressure did not consider influences of the atmospheric pressure value when diagnosing malfunctions to the negative pressure that reaches in the system. Therefore, no trouble will occur when the internal combustion engine is always used under the constant atmospheric pressure, but if the atmospheric pressure is not constant, an erroneous diagnosis may be made.
In other words, internal combustion engines, for automobiles, etc., are driven at high and low altitudes. In such cases, as shown in FIG. 9, the atmospheric pressure values are low at high altitudes and are high at low altitudes.
When the atmospheric pressure value is different as mentioned above, if the purge control valve is opened for the same period because the quantity of purge flow is the same, the target negative pressure does not always reach the predetermined value. Thus, an erroneous diagnosis may be made. This will be a problem regardless of whether a tank internal pressure control valve is provided or not.